EP1578174A1 - Organisches elektrolumineszenzelement und organische elektrolumineszenzanzeige - Google Patents

Organisches elektrolumineszenzelement und organische elektrolumineszenzanzeige Download PDF

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Publication number
EP1578174A1
EP1578174A1 EP02790864A EP02790864A EP1578174A1 EP 1578174 A1 EP1578174 A1 EP 1578174A1 EP 02790864 A EP02790864 A EP 02790864A EP 02790864 A EP02790864 A EP 02790864A EP 1578174 A1 EP1578174 A1 EP 1578174A1
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Prior art keywords
carrier transportation
transportation layer
layer
light emission
cathode
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English (en)
French (fr)
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Yuichiro FUJITSU LIMITED ITAI
Masaya FUJITSU LIMITED NAKAYAMA
Masaru FUJITSU LIMITED KINOSHITA
Jun Fujitsu Limited Kodama
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Fujifilm Corp
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Fujitsu Ltd
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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/06—Luminescent, e.g. electroluminescent, chemiluminescent materials containing organic luminescent materials
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B33/00—Electroluminescent light sources
    • H05B33/12—Light sources with substantially two-dimensional radiating surfaces
    • H05B33/14—Light sources with substantially two-dimensional radiating surfaces characterised by the chemical or physical composition or the arrangement of the electroluminescent material, or by the simultaneous addition of the electroluminescent material in or onto the light source
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K50/00—Organic light-emitting devices
    • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
    • H10K50/14—Carrier transporting layers
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K2211/00—Chemical nature of organic luminescent or tenebrescent compounds
    • C09K2211/10—Non-macromolecular compounds
    • C09K2211/1003—Carbocyclic compounds
    • C09K2211/1014—Carbocyclic compounds bridged by heteroatoms, e.g. N, P, Si or B
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K59/00—Integrated devices, or assemblies of multiple devices, comprising at least one organic light-emitting element covered by group H10K50/00
    • H10K59/10—OLED displays
    • H10K59/17—Passive-matrix OLED displays
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/60—Organic compounds having low molecular weight
    • H10K85/631—Amine compounds having at least two aryl rest on at least one amine-nitrogen atom, e.g. triphenylamine
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/60—Organic compounds having low molecular weight
    • H10K85/631—Amine compounds having at least two aryl rest on at least one amine-nitrogen atom, e.g. triphenylamine
    • H10K85/633—Amine compounds having at least two aryl rest on at least one amine-nitrogen atom, e.g. triphenylamine comprising polycyclic condensed aromatic hydrocarbons as substituents on the nitrogen atom

Definitions

  • the present invention relates to an organic electroluminescent device and an organic electroluminescent display, particularly, to an organic electroluminescent device in which plural carrier transportation layers are provided between an anode and a cathode, and an organic electroluminescent display.
  • an organic electroluminescent device (below, an organic electroluminescent device is referred to as "an organic EL device") is attracting attention, and study and development are being made of the organic EL device as a light emitting source of the next generation, because the organic EL device can be made small easily, has low power consumption, can be fabricated to be a surface light source, requires a much lower power voltage than that of a liquid crystal device, and hence can be used in a flat display and other various display devices.
  • FIG. 1 is a cross-sectional view of an organic EL device of the related art.
  • an organic EL device 10 of the related art includes an anode 12 formed from transparent ITO (Indium Tin Oxide), a hole transportation layer 13, a light emission layer 14, an electron transportation layer 15, and a cathode 16; these layers are stacked sequentially on a transparent glass substrate 11.
  • the hole transportation layer 13 transports holes to the light emission layer 14 efficiently, increases spatial electron density, and increases light emission efficiency.
  • FIG. 2 is an energy diagram of the organic EL device 10 in FIG. 1.
  • holes 22 move from the anode 12 to the light emission layer 14. Due to movement of the holes 22, an electric current of the holes 22 is generated, and due to movement of electrons 21, an electric current of the electrons 21 is generated.
  • the electrons 21 and the holes 22 arrive at the light emission layer 14, the electrons 21 and the holes 22 re-combine, and release energy. Due to the released energy, organic fluorescent materials in the light emission layer 14 are excited and emit light.
  • the efficiency of the light emission of the light emission layer 14 is determined by the magnitudes of the hole current and the electron current, which flow through the organic EL device 10, and when either of the currents is large, the light emission efficiency of the light emission layer 14 declines.
  • the magnitudes of the hole current and the electron current depend on types of the films constituting the organic EL device and film thicknesses of the stacked films. For example, if the magnitude of the hole current is unduly larger than the electron current, there arises a problem in that an increasing portion of the hole current is consumed but does not contribute to light emission, and this results in reduction of the light emission efficiency of the light emission layer 14, and an increase of power consumption.
  • an organic electroluminescent device able to control the magnitude of a hole current and the magnitude of an electron current flowing through a light emission layer, thus enabling efficient re-combination of the holes and the electrons, and hence being superior in light emission efficiency; and an organic electroluminescent display.
  • a specific object of the present invention is to provide an organic electroluminescent device, comprising: a light emission layer formed between an anode and a cathode that face each other; a first carrier transportation layer formed on a side of the light emission layer facing the anode; and a second carrier transportation layer formed on a side of the first carrier transportation layer facing the cathode and in contact with the first carrier transportation layer, wherein an ionization potential Ip P1 of the first carrier transportation layer and an ionization potential Ip P2 of the second carrier transportation layer satisfy Ip P1 ⁇ Ip P2 .
  • the ionization potential is a difference between the valence electron level of the light emission layer or the carrier transportation layer and the energy of the vacuum level, and it has a positive value.
  • a first carrier transportation layer is formed on a side of the light emission layer facing the anode
  • a second carrier transportation layer which has an ionization potential Ip P2 greater than an ionization potential Ip P1 of the first carrier transportation layer, is formed on a side of the first carrier transportation layer facing the cathode.
  • Another specific object of the present invention is to provide an organic electroluminescent device, in which an energy gap Eg EML of the light emission layer and an energy gap Eg P2 of the second carrier transportation layer satisfy Eg EML ⁇ Eg P2 .
  • the energy gap Eg is the difference between the conduction level and the energy of the valence electron level (the energy of the upper edge of the valence electron band), and it has a positive value.
  • the second carrier transportation layer which has an energy gap Eg P2 greater than the energy gap Eg EML of the light emission layer, on a side of the light emission layer facing the anode, recombination of holes and electrons in the second carrier transportation layer is suppressed, and this increases the light emission efficiency of the light emission layer.
  • Another specific object of the present invention is to provide an organic electroluminescent device, comprising a light emission layer formed between an anode and a cathode that face each other; a first carrier transportation layer formed on a side of the light emission layer facing the anode; a second carrier transportation layer formed on a side of the first carrier transportation layer facing the cathode and in contact with the first carrier transportation layer; a third carrier transportation layer formed on a side of the light emission layer facing the cathode; and a fourth carrier transportation layer formed on a side of the third carrier transportation layer facing the cathode and in contact with the third carrier transportation layer, wherein an ionization potential Ip P1 of the first carrier transportation layer and an ionization potential Ip P2 of the second carrier transportation layer satisfy Ip P1 ⁇ Ip P2 , and an electron affinity Ea P3 of the third carrier transportation layer and an electron affinity Ea P4 of the fourth carrier transportation layer satisfy Ea P3 ⁇ Ea P4 .
  • the electron affinity is the difference between a conduction level of the light emission layer or the carrier transportation layer (the energy of the lower edge of the conduction band) and the energy of the vacuum level, and it has a positive value.
  • a second carrier transportation layer which has an ionization potential Ip P2 greater than an ionization potential Ip P1 of the first carrier transportation layer, is formed on a side of the first carrier transportation layer facing the cathode and in contact with the first carrier transportation layer.
  • an energy barrier is built on the interface of the first carrier transportation layer and the second carrier transportation layer, which are in the middle of the path of holes moving from the anode toward the light emission layer.
  • a fourth carrier transportation layer which has an electron affinity Ea P4 greater than an electron affinity Ea P3 of the third carrier transportation layer, is formed on a side of the third carrier transportation layer facing the cathode and in contact with the third carrier transportation layer, an energy barrier is built on the interface of the fourth carrier transportation layer and the third carrier transportation layer, which are in the middle of the path of electrons moving from the cathode toward the light emission layer. As a result, magnitudes of the hole current and the electron current are adjusted, and this increases the light emission efficiency.
  • Another specific object of the present invention is to provide an organic electroluminescent device, in which an energy gap Eg EML of the light emission layer, an energy gap Eg P2 of the second carrier transportation layer, and an energy gap Eg P3 of the third carrier transportation layer satisfy Eg EML ⁇ Eg P2 , and Eg EML ⁇ Eg P3 .
  • a second carrier transportation layer which has an energy gap Eg P2 greater than an energy gap Eg EML of the light emission layer, is formed on a side of the light emission layer facing the anode
  • a third carrier transportation layer which has an energy gap Eg P3 greater than the energy gap Eg EML of the light emission layer, is formed on a side of the light emission layer facing the anode, recombination of holes and electrons in the second carrier transportation layer and the third carrier transportation layer is suppressed, and this increases the light emission efficiency of the light emission layer.
  • Another specific object of the present invention is to provide an organic electroluminescent display comprising an organic electroluminescent device, said organic electroluminescent device including a light emission layer formed between an anode and a cathode that face each other; a first carrier transportation layer formed on a side of the light emission layer facing the anode; a second carrier transportation layer formed on a side of the first carrier transportation layer facing the cathode and in contact with the first carrier transportation layer; wherein an ionization potential Ip P1 of the first carrier transportation layer and an ionization potential Ip P2 of the second carrier transportation layer satisfy Ip P1 ⁇ Ip P2 .
  • a first carrier transportation layer is formed on a side of the light emission layer facing the anode
  • a second carrier transportation layer which has an ionization potential Ip P2 greater than an ionization potential Ip P1 of the first carrier transportation layer, is formed on a side of the first carrier transportation layer facing the cathode.
  • FIG. 3 is a cross-sectional view of an organic EL device according to a first embodiment of the present invention.
  • an organic EL device 30 includes an anode 34, a first carrier transportation layer 35A, a second carrier transportation layer 35B, a light emission layer 36, a third carrier transportation layer 37A, a fourth carrier transportation layer 37B, an electron transportation layer 38, and a cathode 39, and these layers are stacked sequentially on a glass substrate 33.
  • the first carrier transportation layer 35A is formed from a material having the ionization potential Ip P1 lower than the ionization potential Ip P2 of the second carrier transportation layer 35B, and the third carrier transportation layer 37A is formed from a material having the electron affinity Ea P3 less than the electron affinity Ea P4 of the fourth carrier transportation layer 37B.
  • the film thickness of the organic EL device 30 is from 10 nm to 1000 nm. If the film thickness of the organic EL device 30 is greater than 1000 nm, the resistance increases and this impedes current conduction. If the film thickness of the organic EL device 30 is less than 10 nm, the light emission layer 36 becomes too thin, and this results in insufficient light emission luminance.
  • the anode 34 and the cathode 39 are used for applying appropriate voltages to the organic EL device 30.
  • the anode 34 is formed from ITO or IZO (Indium Zinc Oxide) and the like
  • the cathode 39 is formed from an Al/LiF electrode or Ag/Mg electrode or the like.
  • the first carrier transportation layer 35A and the second carrier transportation layer 35B are respectively formed from materials having high electron and hole transportation capabilities.
  • the first carrier transportation layer 35A may be formed from 2-TNATA (produced by Toyo Ink Co.), represented by the following chemical formula (1).
  • the second carrier transportation layer 35B may be formed from ⁇ NPD (produced by Toyo Ink Co.), represented by the following chemical formula (2), or EL002 (produced by Hodogaya Chemistry Co.), or the like.
  • the film thicknesses of the first carrier transportation layer 35A and the second carrier transportation layer 35B are set to be in a range from 10 nm to 50 nm, which can be appropriately selected according to the number of layers included in the organic EL device 30 and the total thickness of the organic EL device 30.
  • the third carrier transportation layer 37A and the fourth carrier transportation layer 37B are respectively formed from materials having high electron and hole transportation capabilities.
  • the third carrier transportation layer 37A may be formed from TYE-704 (produced by Toyo Ink Co.).
  • the fourth carrier transportation layer 37B may be formed from TYG-201 (produced by Toyo Ink Co.), or CBP (produced by Dojindo Chemistry Institute), or the like.
  • the film thicknesses of the third carrier transportation layer 37A and the fourth carrier transportation layer 37B are set to be in a range from 10 nm to 50 nm, which can be appropriately selected according to the number of layers included in the organic EL device 30 and the total thickness of the organic EL device 30.
  • the light emission layer 36 includes materials which emit light of desired colors, for example, organic fluorescent materials, and may be formed from a metal complex, such as TYG-201 (produced by Toyo Ink Co.), or may be formed from Alq 3 (tris (8-hydroxyquinolio) aluminum) (produced by Toyo Ink Co.), represented by the following chemical formula (3).
  • the film thickness of the light emission layer 36 is set to be in a range from 10 nm to 50 nm, which can be appropriately selected in the same way as the aforesaid carrier transportation layers.
  • the electron transportation layer 38 is formed from materials having high electron transportation capability, and for example, the electron transportation layer 38 may be formed from TYE-704 (produced by Toyo Ink Co.) or TYE-703 (produced by Toyo Ink Co.).
  • the above layers of the organic EL device 30 are formed by vacuum evaporation with the pressure to be 1.33 x 10 -4 Pa, and the temperature of the glass substrate 33 to be the room temperature.
  • FIG. 4 is an energy diagram of the organic EL device 30 according to the first embodiment of the present invention.
  • Ea, Eg, and Ip respectively represent the electron affinity Ea, the energy gap Eg, and the ionization potential Ip of each of the layers of the organic EL device 30.
  • the electron affinity Ea is the difference between a conduction level 41 (the energy of the lower edge of the conduction band) and the energy of the vacuum level
  • the energy gap Eg is the difference between the conduction level 41 and the energy of the valence electron level 42 (the energy of the upper edge of the valence electron band)
  • the ionization potential Ip is the difference between the valence electron level 42 and the energy of the vacuum level.
  • the second carrier transportation layer 35B is formed, and on the side of the second carrier transportation layer 35B facing the anode 34, the first carrier transportation layer 35A is formed. Further, on the side of light emission layer 36 facing the cathode 39, the third carrier transportation layer 37A is formed, and on the side of the third carrier transportation layer 37A facing the cathode 39, the fourth carrier transportation layer 37B is formed.
  • the ionization potential of the first carrier transportation layer 35A and the ionization potential of the second carrier transportation layer 35B are represented by Ip 35A and Ip 35B , respectively, it is set that the relation between the first carrier transportation layer 35A and the second carrier transportation layer 35B satisfies Ip 35A ⁇ Ip 35B .
  • Eg 36 , Eg 35B , and Eg 37A energy gaps of the light emission layer 36, the second carrier transportation layer 35B, and the third carrier transportation layer 37A are represented by Eg 36 , Eg 35B , and Eg 37A , respectively. It is set that the relations between the light emission layer 36, the second carrier transportation layer 35B, and the third carrier transportation layer 37A satisfy Eg 36 ⁇ Eg 35B , and Eg 36 ⁇ Eg 37A .
  • the current of the holes is explained below.
  • the holes moves from the anode 34 toward the cathode 39 through the first carrier transportation layer 35A and the second carrier transportation layer 35B, and arrive at the light emission layer 36.
  • an energy barrier BH-A is built on the interface of the first carrier transportation layer 35A and the second carrier transportation layer 35B.
  • the holes are stored in the first carrier transportation layer 35A because of the energy barrier BH-A, and the magnitude of the hole current decreases.
  • the magnitude of the hole current flowing through the light emission layer 36 is adjusted, and the electrons and the holes are re-combined after the adjustment. This increases the efficiency of light emission.
  • the energy gap Eg 35B of the second carrier transportation layer 35B is greater than the energy gap Eg 36 of the light emission layer 36, recombination in the second carrier transportation layer 35B is prevented, and this increases the efficiency of light emission in the light emission layer 36.
  • E BE-B Ea 37B - Ea 37A . Because it is set that Ea 37A ⁇ Ea 37B , it is obtained that E BE-B > 0.
  • the electrons are stored in the fourth carrier transportation layer 37B, and the magnitude of the electron current decreases. Thereby, the magnitude of the electron current flowing through the light emission layer 36 can be adjusted.
  • plural energy barriers BH-A related to the holes are formed, and by providing the energy barrier BE-B related to the electrons, the magnitudes of the hole current and the electron current can be adjusted, and this increases the efficiency of light emission.
  • the energy gap Eg 37A of the third carrier transportation layer 37A is greater than the energy gap Eg 36 of the light emission layer 36, recombination in the third carrier transportation layer 37A is prevented, and this increases the efficiency of light emission in the light emission layer 36.
  • the present embodiment it is possible to prevent light emission in the second carrier transportation layer 35B which is formed on the side of light emission layer 36 facing the anode 34 and in the third carrier transportation layer 37A which is formed on the side of the light emission layer 36 facing the cathode 39, and at the same time, it is possible to adjust the magnitudes of the hole current and the electron current and increase the light emission efficiency in the light emission layer 36.
  • the organic EL device according to the first embodiment was fabricated in the following way.
  • an ITO electrode as the anode 34
  • a 2-TNATA layer film thickness: 30 nm/a ⁇ NPD layer
  • a 2-TNATA layer film thickness: 10 nm/a ⁇ NPD layer
  • a 2-TNATA layer film thickness: 10 nm/a ⁇ NPD layer
  • a 2-TNATA layer film thickness: 10 nm/a ⁇ NPD layer (film thickness: 30 nm) as the first carrier transportation layer 35A / the second carrier transportation layer 35B
  • a non-doped TYG-201 layer having a film thickness of 20 nm and emitting green light as the light emission layer 36
  • a TYE-704 layer film thickness: 10 nm)/a TYG-201 layer (film thickness: 10 nm)
  • a TYE-704 layer film thickness: 10 nm)/a TYG-201 layer
  • first carrier transportation layer 35A and the second carrier transportation layer 35B are formed on the side of light emission layer 36 facing the anode 34, but in the first example of the present embodiment, three stacked layers were formed.
  • only one stacked layer of the third carrier transportation layer 37A and the fourth carrier transportation layer 37B is formed on the side of light emission layer 36 facing the cathode 39, but in the first example of the present embodiment, two stacked layers were formed.
  • a spectrophotometer U-4100 produced by Hitachi Co.
  • FIG. 5 shows dependence of the optical absorption spectrum on the wavelength.
  • a curve J indicates measurement results of the optical absorption spectrum.
  • the region T corresponds to a straight line portion of the curve J when the strength shown in the optical absorption spectrum rises.
  • the region U corresponds to a straight line portion of the curve J in the un-absorbed wavelength region in the optical absorption spectrum.
  • a straight line K is drawn to overlap on the curve J in the region T.
  • a straight line L is drawn to overlap on the curve J in the region U.
  • the energy gap Eg is found from a cross-point of the straight line K and the straight line L.
  • the thin films which are formed in the same way as those used in the measurement of the energy gap Eg, were irradiated in the air by ultraviolet rays, and an atmospheric ultraviolet photo-electron spectrometer, for example, AC-1 fabricated by RIKEN KEIKI, is used to measure the number of the emitted photo-electrons.
  • the ionization potential Ip is deduced from a relation between the energy of the ultraviolet rays and the root of the number of the emitted photo-electrons.
  • the measurement conditions of the AC-1 fabricated by RIKEN KEIKI are as follows.
  • the energy range of the ultraviolet rays is from 3.8 to 6.2 eV
  • the strength of the ultraviolet rays is 20 nW
  • the thickness of the thin films is 50 nm.
  • FIG. 6 shows a relation between the energy of the ultraviolet rays and the root of the number of the emitted photo-electrons.
  • the number of the emitted photo-electrons depends on the energy of the ultraviolet rays. As illustrated in FIG. 6, in a region N, because the energy of the ultraviolet rays is small, no photo-electrons are emitted, but in a region O, since the energy of the ultraviolet rays is sufficiently large, photo-electrons are excited to levels at energies higher than the vacuum levels, and photo-electrons are emitted.
  • a straight line P in FIG. 6 is obtained by least square approximation in a range of the ultraviolet rays' energy from 4.2 eV to 5.3 eV.
  • a straight line Q is obtained by least square approximation in a range of the ultraviolet rays' energy from 5.6 eV to 5.9 eV.
  • the cross point R of the straight line P and the straight line Q represents threshold energy of photo electron emission, and this threshold energy of photo electron emission corresponds to the ionization potential Ip.
  • the electron affinity Ea is calculated from the difference between the ionization potential Ip and the energy gap Eg.
  • FIG. 7 is a table summarizing the electron affinity Ea, the ionization potential Ip, and the energy gap Eg of the layers shown in the first example of the present embodiment.
  • the ionization potential Ip of the 2-TNATA layer as the first carrier transportation layer 35A is 5.19 eV
  • the ionization potential Ip of the ⁇ NPD layer as the second carrier transportation layer 35B is 5.46 eV.
  • the electron affinity Ea of the TYG-201 layer as the fourth carrier transportation layer 37B is 3.20 eV
  • the electron affinity Ea of the TYE-704 layer as the third carrier transportation layer 37A is 2.97 eV.
  • the holes are stored in the 2-TNATA layers as the first carrier transportation layer 35A, and the magnitude of the hole current decreases.
  • the two energy barriers formed on the interface of the fourth carrier transportation layer 37B and the third carrier transportation layer 37A electrons are stored in the TYG-201 layers as the fourth carrier transportation layer 37B, and the magnitude of the electron current decreases.
  • the magnitudes of the hole current and electron current flowing through the light emission layer 36 are adjusted, and the electrons and the holes are re-combined after the adjustment. This increases the efficiency of light emission.
  • the energy gap Eg of the ⁇ NPD layer as the second carrier transportation layer 35B is 3.04 eV
  • the energy gap Eg of the TYE-704 as the third carrier transportation layer 37A is 2.76 eV
  • the energy gap Eg of the light emission layer 36 is 2.40 eV.
  • the probability of electron-hole re-combination increases, and this increases the light emission efficiency of the TYG-201 layer used for the light emission layer 36.
  • FIG. 8 is a table for explaining the layer structure of an organic EL device as the second example of the present embodiment.
  • the number of the stacked structures of the first carrier transportation layer 35A and the second carrier transportation layer 35B between the light emission layer 36 and the anode 34 is reduced by one compared with that in the first example of the present embodiment. Thickness of each layer can be appropriately determined according to the total thickness of the organic EL device.
  • the number of the stacked structures of the first carrier transportation layer 35A and the second carrier transportation layer 35B is less than that in the first example by one.
  • the magnitudes of the hole current and the electron current depend on types of the films constituting the organic EL device and film thicknesses of the stacked films.
  • the hole current is too small and the light emission efficiency declines. Therefore, by reducing the number of the energy barriers against the holes by one, reduction of the hole current is suppressed more effectively than in the first example of the present embodiment. Thereby, more electrons and holes are re-combined, and this increases the light emission efficiency of the whole organic EL device.
  • the organic EL device as the first example for comparison was fabricated in the following way.
  • On a glass substrate by vacuum evaporation, there were stacked sequentially an ITO electrode as an anode, a 30 nm thick 2-TNATA layer as a hole injection layer, a 50nm thick ⁇ NPD layer as a hole transportation layer, a non-doped TYG-201 layer having a film thickness of 50 nm and emitting green light as the light emission layer, a TYE-704 layer having a film thickness of 40 nm as an electron transportation layer, and an Al/LiF stacked film including a 0.5 nm thick LiF film and a 100 nm Al film as a cathode.
  • the pressure inside the vacuum evaporation apparatus was 1.33 x 10 -4 Pa, and the temperature of the glass substrate was room temperature.
  • the organic EL devices as the second and third examples for comparison were fabricated in the same way as the above first example of the organic EL device for comparison.
  • FIG. 9 is a table for explaining the layer structure of the organic EL devices as the second and third examples for comparison.
  • FIG. 10 shows a relation between the hole current and the voltage of the organic EL device as the first example of the present embodiment.
  • FIG. 11 shows a relation between the hole current and the voltage of the organic EL device as the second example of the present embodiment.
  • FIG. 12 shows a relation between the hole current and the voltage of the organic EL device as the first example for comparison.
  • the organic EL device as the first example of the present embodiment as illustrated in FIG. 10
  • three energy barriers are formed on the anode side of the light emission layer
  • two energy barriers are formed on the anode side of the light emission layer.
  • one energy barrier is formed in the organic EL device as the first example for comparison. From FIG. 10 through FIG. 12, it is clear that the hole current decreases along with more energy barriers being formed on the anode side of the light emission layer.
  • FIG. 13 shows a relation between the electron current and the voltage of the organic EL device as the first example of the present embodiment.
  • FIG. 14 shows the dependence of the light emission efficiency of the organic EL device on the voltage. As illustrated in FIG. 14, the light emission efficiency of the organic EL device as the second example of the present embodiment has the highest light emission efficiency.
  • the light emission efficiency of the organic EL device in the second example of the present embodiment is higher than the light emission efficiency of the organic EL device in the first example of the present embodiment, in which more energy barriers against holes are formed.
  • the magnitude of the electron current is determined by the number and mobility of the electrons
  • the magnitude of the hole current is determined by the number and mobility of the holes.
  • the mobility of electrons is less than the mobility of holes, even if the number of electrons is the same as the number of holes, the magnitude of the electron current does not equal the magnitude of the hole current. If energy barriers are not formed, the magnitude of the hole current is greater than the magnitude of the electron current, whereas if energy barriers against holes are present, the magnitude of the hole current decreases, and the number of holes also decreases.
  • the number of holes that recombine with electrons is unduly reduced.
  • the second example of the present embodiment since two energy barriers against holes are formed, which is less than that in the first example, it is considered that the difference between the number of holes and the number of electrons is reduced, and the light emission efficiency of the organic EL device is increased, resulting in better results than in the first example of the present embodiment.
  • FIG. 15 is a perspective view showing a schematic configuration of an organic electroluminescent display (abbreviated to be "organic EL display” below) according to a second embodiment of the present invention.
  • organic EL display As illustrated in FIG. 15, in an organic EL display 50, Al/LiF electrodes as the cathode 39, and ITO electrodes as the anode 34 are formed on a glass substrate 33 so as to intersect with each other at right angles, and between the cathode 39 and the anode 34, there is provided a stacked structure 45 forming the organic EL device of the first embodiment.
  • the organic EL display 50 emits light by applying specified voltages on the ITO electrodes and the Al/LiF electrodes corresponding to regions of the light emission layer to be flashed. Due to such a configuration of the organic EL display 50, the light emission efficiency can be improved.
  • the numbers of the energy barriers against holes and the energy barriers against electrons shown in the first example of the first embodiment may be appropriately selected for individual organic EL device.
  • the present invention by providing a first carrier transportation layer on a side of the light emission layer facing the anode, and a second carrier transportation layer, which has an ionization potential Ip P2 less than an ionization potential Ip P1 of the first carrier transportation layer, on a side of the first carrier transportation layer facing the cathode, an energy barrier is built on the interface of the first carrier transportation layer and the second carrier transportation layer, which are in the middle of the path of holes moving from the anode toward the light emission layer.
  • the holes are stored in the first carrier transportation layer, and the magnitude of the hole current flowing through the light emission layer decreases. As a result, the magnitude of the current of the holes is adjusted, and this increases the light emission efficiency.

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  • Chemical & Material Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Electroluminescent Light Sources (AREA)
EP02790864A 2002-12-25 2002-12-25 Organisches elektrolumineszenzelement und organische elektrolumineszenzanzeige Withdrawn EP1578174A1 (de)

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US20050104084A1 (en) 2005-05-19
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US7233105B2 (en) 2007-06-19
JP4263175B2 (ja) 2009-05-13

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